This invention relates to magnesium additives for ferrous melts.
It is known that magnesium may be incorporated in molten iron and steel in order to remove undesired sulphur from the melt and to convert the iron to nodular form on casting. However magnesium has a boiling point of 1107.degree. C. which is substantially lower than the melting points of iron alloys and consequently the uncontrolled addition of magnesium metal to a ferrous melt produces magnesium vapour which escapes from the melt and burns on contact with the atmosphere. Most of the magnesium is thus wasted and the burning vapour constitutes a serious hazard.
U.S. Pat. No. 3,957,502 discloses a magnesium based additive for a ferrous melt and particles of a refractory material inert to magnesium at the melting point of the latter, the structure being such that the refractory material forms a coherent metal-permeable matrix around the magnesium particles when subjected to the temperature of a ferrous melt. The constituents of the additive may be aggregated to form briquettes. It is mentioned that calcium fluoride may be added, preferably in quantities in excess of 10%, to improve the efficiency of utilisation of the magnesium. Magnesium should react with calcium fluoride at an elevated temperature to produce involatile magnesium fluoride and elementary calcium which like magnesium reacts with sulphur, to form calcium sulphide, which is thus removed from the melt. However calcium has a boiling point in the region of 1500.degree. C. and so little calcium is likely to leave the melt as vapour.
It has now been found that the use of briquettes which contain more than 50% by weight of magnesium, while giving a more moderate reaction than pure magnesium, still give a quite violent emission of magnesium vapour. It has also been found that an increased content of calcium fluoride for a given proportion of magnesium gives a more violent reaction, apparently because the calcium fluoride acts as a flux which has the effect of causing progressive fusion of the surface layers of the briquettes in contact with the molten ferrous melt. If a high content of calcium fluoride is used the matrix is destroyed by this melting before the magnesium has fully diffused into the melt. On the other hand, it is desirable that some fluxing action should take place to decompose the matrix after the reaction to avoid the presence, on the surface of the melt, of residual lumps of material. Eventual decomposition of the matrix is particularly desirable when the additive is added to the melt by plunging, in which case it is pushed under the surface of the melt in a bell. Residual matrix material may stick to the underside of the bell, which consequently requires repair or cleaning; the removal of the matrix material by fluxing helps to avoid this occurrence. A calcium fluoride content from 1 to 10% enables a suitable rate of decomposition of the matrix to be achieved.